I thought I would share an experimental fork of duplicacy https://github.com/dupluxy/dupluxy that features support for hard links, special files, slightly better xattr handling on Linux and MacOS (though this still needs more thought/work) and even the nasty BSD style/chflags file flags.
I wanted to answer the question if it would be feasible to generate snapshots that are still processable by duplicacy but can support all the above features. The answer is yes with the caveat it obviously can’t be guaranteed that future versions of duplicacy would not make some assumption breaking changes, though this seems unlikely.
Because of the way file metadata is encoded in the snapshot there isn’t a clean way to extend, so this method uses some hacks, clever tricks, to pack the necessary data in a way that doesn’t break duplicacy and seems unlikely to do so in the future. More snapshot detail here.
The usefulness of hardlinks should be obvious. Why special files? Backing up /dev is pretty useless, but the overlay2 filesystem on Linux often used for containers uses char special inodes to mark upper layer “whiteout entries”. BSD style file flags are used for marking compression/COW attributes on btrfs for one, in addition to the other stuff in the chflags man page.